The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
One process that can be used to produce purified process water is membrane distillation. Membrane distillation is a separation method in which a porous membrane is used to separate a vapour phase from a hot vapourising liquid feed (or retentant) on one side of the membrane and pass that vapour to a cold condensing, permeate fluid, or in some cases a cold surface, on the other side. The driving force for the diffusion is a vapour pressure difference created by the temperature difference across the membrane or reduced vapour pressure on the permeate side. Separation is achieved utilising the relative volatility of various components in the vapourising fluid enabling vapour from components in the vapourising fluid of higher volatility to pass through the membrane pores by a convective or diffusive mechanism. The retained components remain in the vapourising fluid by the hydrophobicity of the membrane material which is a barrier to the liquid phase but allows vapour to pass through the membrane. In any case, the vapour penetrates through the porous membrane, and condenses in the cooler fluid or surface on the condensing permeate side or in an externally mounted condenser unit. The condensed vapour is therefore removed from the retentant and thus is beneficial in its effect to separate the vapour from the retentant. An example of such benefit is the desalination of saline water.
There are four broad types of membrane distillation systems:                1. Direct contact membrane distillation (DCMD), where both the warm, vapourising stream and the cold condensing stream (distillate stream) are in direct contact with the membrane.        2. Air gap membrane distillation (AGMD), where a condenser surface contacting the condensing stream is separated from the membrane by an air gap.        3. Sweeping gas membrane distillation (SGMD), where the vaporised portion of the vapourising stream is removed in vapour form by an inert gas.        4. Vacuum membrane distillation (VMD), where the vaporised portion of the vapourising stream is removed in vapour form by vacuum.        
Commonly, membrane distillation apparatus employ single membrane layer arrangements. However, when multilayer apparatus are constructed the membranes are usually supported on and between support plates, with a gasket seal set between each plate to create a fluid seal around each membrane layer. Each of the support plates are typically configured with inlets and outlets passages comprising a series of aligned passages and apertures which extend perpendicularly (relative to the longitudinal plane of the membranes and support plates) through the various support plates, seals and other layers of the apparatus. This configuration necessitates the fluid carrying passages of the inlets and outlets to extend through the various sealing gaskets. The resulting fluid carrying aperture in the sealing gasket can compromise sealing integrity between the support plates.
Sealing may be improved through the use of multiple seals and/or stricter and tighter sealing protocols and fastening arrangements between each layer in the stack. However, these multilayer module designs can provide difficulties in assembly and disassembly, for example for the replacement of wetted or fouled membranes. Such arrangements can also result in less optimal distribution of fluid flow to the membrane.
An example of one prior membrane distillation arrangement is provided in Russian Patent Publication RU2040314C1. This patent publication describes a multilayer membrane distillation device comprising a stacked arrangement of heat exchange chambers, membrane distillation inlet chambers and membrane distillation outlet chambers. Each of the heat exchange chambers, membrane distillation inlet chambers and membrane distillation outlet chambers are formed using a stacked arrangement of support frames which include a space therebetween. The support frame of the heat exchange chamber supports and are closed on both sides by heat transfer plates, and the support frame of the membrane distillation chambers support and are closed on both sides with a hydrophobic microporous membrane.
While the use of the described support frames would assist in sealing, the inlet and outlet conduits of the arrangement are still formed by aligned passages and apertures running perpendicularly (relative to the longitudinal length of the membrane and support frames) formed in the various layers of arrangement. Each support frame, and seal include aligned apertures for the fluid flow. Each layer seal also includes complementary fluid flow apertures to accommodate the fluid conduits in the frame. These fluid flow apertures are the weakest areas for sealing, since these portions of the seal are not solid and may be deformed under pressure.
It is also noted that the heat exchange chambers and membrane distillation inlet chambers comprise a split level fluid circuit, in which the hot feed fluid flows through a step structure formed by a partition plate in the chamber. The Applicant considers that the hot feed in this arrangement will likely exchange heat with itself through the partition which results in heat loss as a result of the feed flow in at one side of the partition being at higher temperature to the flow out at the other side of the partition.
It would therefore be desirable to provide an alternate and/or improved membrane distillation arrangement which has improved sealing, ease of assembly particularly between layers in a membrane stack.